A number of microorganisms are known to have beneficial effects on plant growth. Among these are nitrogen fixing bacteria of the Rhizobium species, which are symbionts of leguminous species. Azospirillum species, which are free living nitrogen fixing bacteria associated with the roots of grasses, are also now recognized for their plant growth promoting qualities (Mishustin and Shilnikova, Moscow, Science Publ. House, 1973; Mishustin and Shilnikova, Moscow, Science Publ. House, 1968). More specifically, certain strains of Azospirillum brasilense have been shown to enhance accumulation of various minerals in wheat and soybean (Bashan et al., Applied and Environ. Microbiol., 56(3):769-775 (1990)), increase dry weights of maize shoots (O'Hara et al., Can. J. Microbiol., 27:871-877 (1981), and increase dry weights of sorghum, pearl millet and napier grass (Smith et al., Applied and Environ. Microbiol., 47(6):1331-1336 (1984)).
Inoculation of seeds or soil with beneficial microorganisms, including Azospirillum, for crop improvement has been practiced for a number of years. However, variable and inconsistent results have often been observed possibly due to loss of inoculant viability or variability of dosage due to changes in inoculant viability (Okon et al., CRC Crit. Rev. Biotechnology, 6:61-85 (1987)). Further, the use of specific types of microorganisms as crop inoculants has met with varying degrees of success most likely due to variables that include: (1) the presence or absence of adequate micro- and macro-nutrients in the soil to support the propagation of the microorganisms; (2) the amount of organic material in the soil available to hold nutrient and microbes in the soil and provide a suitable environment for microbial growth; (3) the presence or absence of certain minerals or compounds required by the plant for proper uptake of the nutrients provided by microbial activity; and (4) variations in soil characteristics such as soil type, pH, temperature and moisture.
A number of Azospirillum strains have been isolated from southern areas. These strains seem to have limited adaptive capabilities when introduced into other soil types in different climate zones (Bashan et al., Can. J. Microbiol., 36:591-608 (1990); Nur et al., Can. J. Microbiol., 26(4):482-485 (1980); Smith et al., Applied and Environ. Microbiol., 47(6):1331-1336 (1984); O'Hara et al., Can. J. Microbiol., 27:871-877 (1981)). Further, these types of microorganisms were not very effective simulators of legume-rhizobia symbiosis as they tended to stimulate the growth of plant dry weight and nodule formation, but did not always provide an increase in nitrogenase activity (Bashan et al., Applied and Environ. Microbiol., 56(3):769-775 (1990); Yahalom et al., Applied and Environ. Microbiol., 33(6):510-519 (1987); Schmidt et al., Azospirillum IV: Genetics, Physiology, Ecology, Springer-Verlag, pp. 92-101, (1988)).